1. Field of the Invention
The present invention relates to a circuit using an optical sensor of the charge storage type, which is equipped with a parallel capacitor, for detecting the intensity of light received by the optical sensor in the form of a time signal and, more particularly, to a light intensity detecting circuit suited for accurately detecting the intensity of light received by each of the optical sensors contained in an image sensor.
2. Description of the Prior Art
In order to detect the intensity of light, the prior art has used either an optical sensor with a variety of photoconductive elements such as phototransistors or photodiodes or an image sensor having the optical sensors integrated. As is well known in the art, the intensity of light to be received by the optical sensor can vary over a very wide range, for example a ratio of 1 to 10.sup.6. The output voltage or current obtained from a simple photoelectric element cannot indicate such a widely varying optical intensity. In accordance with the prior art as illustrated in FIG. 7, therefore, an optical sensor of the charge storage type, 11 and 12, is employed. A parallel capacitor 12 acting as a kind of integration element is charged or discharged by a photocurrent proportional to the optical intensity coming from the optical sensor 11 so that the light intensity may be expressed by the time period for which the terminal voltage of the capacitor changes a predetermined amount.
The capacitor 12 to be connected in parallel with the optical sensor 11 may ordinarily have a small capacity, which is frequently exemplified by a stray capacity accompanying the photoelectric element or the junction capacity of its semiconductor junction.
A photodiode may be employed as the optical sensor 11 and, together with parallel capacitor 12 acting as the diode junction capacity, constitutes one charge storage type optical sensor within an image sensor 10. The light sensors 11 in the image sensor 10 each have first terminals connected commonly to receive a fixed potential Vd and second terminals connected with one input of a comparator 2 in a detecting circuit 1 provided for each optical sensor. The one input of the comparator 2 is further connected through a transistor 3 with a first potential V1. The other, or second, input of the comparator 2 is supplied with a second potential V2 as a comparative reference potential and delivers an output signal S. The first potential V1 is most simply exemplified by the ground potential, and the second potential V2 is exemplified by a potential between the ground potential and the fixed potential Vd.
FIG. 8 illustrates the operation of the prior art detecting circuit 1. In order to cause this circuit to start the detecting operation, as shown at (a) in FIG. 8, a reset pulse R is fed to render the transistor 3 conductive so that the potential v of the first input of the comparator 2 is set at the first potential V1, as shown at (b) in FIG. 8. At this time, the parallel capacitor 12 of the photodiode 11 is charged with a voltage between the fixed potential Vd and the first potential V1, and the detecting circuit 1 is subjected to the so-called "initialization" so that a detection signal S or the output of the comparator 2 is reset from "H" to "L", as shown at (c) in FIG. 8. Then, the capacitor 12 is discharged by the photocurrent based on the light L received by the photodiode 11 so that the potential v at the first input of the comparator 2 gradually rises, as shown at (b) in FIG. 8. As easily seen, the gradient or rate of this rise is proportional to the intensity of the light received by the photodiode 11.
When the potential v rises to the second potential V2, the detection signal S from the comparator 2 has its state switched from "L" to "H", as shown at the right of (c) in FIG. 8. The time period Td, for which the detection signal S is at the level "L" after extinction of the reset pulse R, indicates the intensity of light received by the photodiode 11. This time period Td expressed by the detection signal S is naturally inversely proportional to the light intensity, but this inverse proportion need not be elaborately corrected to a strictly proportional relation. The detection signal S is usually used, as it is, as the signal indicating the light intensity.
The light intensity detecting circuit according to the prior art can measure or detect the light intensity accurately on principle in terms of the length of the time period indicated by the detection signal of the optical sensor even if the intensity of light received by the optical sensor varies over a very wide range, as described above.
Despite this advantage, however, the light intensity detecting circuit of the prior art has the disadvantage that a high detection accuracy, even if demanded, is restricted by a few factors. One of these restricting factors is the dark current of the optical sensor. In most of the optical sensors, as is well known in the art, a kind of leakage current known as the dark current will flow even if no light is received. The charge storage type of optical sensor is especially liable to have its detection accuracy deteriorated in the range of low light intensity. In the example of FIG. 7, the parallel capacitor 12 is excessively discharged by the dark current during the time period for the light detection such that the detection time period expressed by the detection signal S is shortened accordingly.
This behavior due to the dark current is illustrated in FIG. 9. The broken line in FIG. 9 indicates the rise of the potential v when there is no dark current. The point of intersection of this broken line with the second potential V2 indicates the time period Tr, which in turn indicates the true detection time period expressing the intensity of light actually received by the photodiode 11. Due to the presence of the dark current, however, the rise of the potential v follows the solid line, as shown in FIG. 9, which determines the detection time period Td by its point of intersection with the second potential. This detection time period Td is always slightly shorter than the true detection time period Tr, and the difference leads to the detection error. The value of the dark current is naturally very small, for example 0.5 to 0.6 pA at most, in the case in which the optical sensor is a photodiode. However, the error cannot be neglected, as will be easily understood, if the light intensity is so weak that the detection time period is elongated.
Another restricting factor of the prior art light intensity detecting circuit is an error or offset in the operation of the comparator. In the example of FIG. 7, the comparator 2 must operate to change its output state when the value of the potential v becomes exactly equal to the second potential V2 acting as the comparative reference potential. As a practical matter, however, the comparator 2 has an operating potential, which fluctuates within the range of high and low offsets of the comparative reference potential. These offsets are usually small, within .+-.1% or less of the comparative reference potential and about .+-.0.01 V in voltage. Even in this usual case, however, the error cannot be ignored for a long detection time period. FIG. 9 shows the effect of the operational offsets of the comparator 2 which are indicated by .DELTA.V. As shown, the detection time period Td expressed by the detection signal S may fluctuate between the minimum Tn and the maximum Tx by the offsets.